The present invention concerns the control strategies implemented in powertrain computation units.
It finds a preferred but non-limiting application to powertrains comprising at least one internal combustion engine and one electrical traction machine.
To be more precise, an object of this invention is a method for controlling state changes of a vehicle drivetrain connecting at least one internal combustion engine and/or one electrical machine to the wheels of the vehicle via a transmission that transfers torque from the internal combustion engine and/or the electrical machine to the wheels in accordance with one or more gear ratios.
These states are defined by various combinations of couplers and reducers to transfer torque from the internal combustion engine and/or the electrical machine to the wheels in accordance with one or more gear ratios. The state of a drivetrain can therefore be defined by a combination of solicited couplers and reducers. The state target of the drivetrain aims to optimize the operating point of the powertrain. In an internal combustion powertrain, a drivetrain state can simply be defined by the engagement of a ratio and the position (open or closed) of an input clutch between the engine and the gearbox. On a hybrid powertrain, its definition is necessarily more complex because it has to integrate the state of one or more electrical machines that are able to propel the vehicle via the same axle as the internal combustion engine or via another axle.
On a hybrid vehicle, the acoustic behavior of the vehicle depends, inter alia, on the distribution between electrical power and internal combustion power. Harshness, linked to the performance of the powertrain, also depends on the state of charge of the traction battery, as only the internal combustion engine is usable when it is discharged. Finally, energy management laws that determine the distribution of power in compliance with consumption and pollution reduction constraints at each operating point. Similarly, a classification of the optimum states of the drivetrain must also take account of the state of charge of the battery.
The drivetrains of a internal combustion vehicle and a hybrid vehicle also differ considerably:                on a hybrid vehicle, the internal combustion engine is not the only source of motor power;        for the same power demand, there is a plurality of possible combinations of the power delivered by the internal combustion engine and that delivered by the electric motor(s);        depending on the technical definition envisaged, power from the electrical machine either passes through the transmission or does not;        the static and dynamic max/min limitations of the hybrid powertrain depend on the state of charge of the battery and so vary over time;        the electric or ZEV (zero emission vehicle) traction mode combines one or more specific states of the drivetrain by the same token as the discrete ratios of the internal combustion engine.        
Generally speaking, acoustic phenomena, harshness and likewise the consumption and pollution reduction level address particular constraints on a hybrid vehicle. For the same operating point (speed, motor power), the acoustic level of the powertrain depends on the distribution between electric power and internal combustion power, the electric motor being quieter.
Harshness, which is linked to the performance of the powertrain, depends on the state of charge of the battery. If it is charged, it is possible to use simultaneously the power delivered by the electric motor and by the internal combustion engine. If it is discharged, the overall power available is reduced as the only source of energy available is the internal combustion engine, with a possible reduction of performance. Finally, the consumption and pollution reduction requirements are taken into account in energy management laws that in each hybrid state establish the distribution between the power delivered by the internal combustion engine and that delivered by the electrical machines as a function of the state of charge of the battery.
Moreover, depending on the type of hybrid architecture, the sources of motor power can be installed in the vehicle in various configurations. For example, the electrical machine(s) are associated with the rear wheels, the crankshaft, the secondary shaft of the gearbox, etc. However, the states of the drivetrain of any hybrid vehicle are defined as combinations of the traction units via or not via a transmission with a plurality of ratios. Two target states X and Y can therefore be arrived at in different ways from a current drivetrain state Z. The state changes result from different mechanical changes in the transmission, for example:                coupling the electrical machine without changing ratio for the internal combustion engine,        changing the ratio of the internal combustion engine with the ratio of the electrical machine being maintained,        changing the ratio of the electrical machine without internal combustion engine, etc.        
Accordingly, a change from a current state Z can proceed with total interruption of the torque at the wheel, a reduction of the level of torque at the wheel or else with no reduction of the level of torque, depending on the target states X or Y, or in accordance with mechanical changes effected by the transmission. Through significant unanticipated reductions of acceleration, reductions or interruptions of torque at the wheel during changes of state are negatively perceived in terms of harshness by the driver and his passengers and degrade performance, such as the “brio” of the vehicle.
The problem is encountered on a hybrid transmission as described in the publication WO2012131259, for example.
This problem is illustrated in the following way in FIG. 1, in which are shown maximum acceleration curves for different kinematic modes of the transmission. If it is considered that at the operating point 1 in the diagram the current state of the drivetrain is an electric ratio termed ZEV1 and that the vehicle has reached its maximum speed in that state, the transmission must switch to another drivetrain state addressing the harshness constraints associated with the point 1 but also the energy optimization of the vehicle. From the point 1, the states termed HYB23, TH2, HYB33 and ZEV3 respectively corresponding to a first hybrid ratio, an internal combustion ratio, a second hybrid ratio and another electric ratio are permissible from the harshness point of view. However, once established they offer different services. Moreover, the changes to HYB23 and TH2 can be achieved without interruption or reduction of the acceleration of the vehicle, whereas the changes to ZEV3 and HYB33 result in a significant reduction in the acceleration of the vehicle because producing those states involves a passage through the neutral state of the transmission involving total interruption of the torque at the wheels. If an energy optimization criterion calls for the ZEV3 or HYB33 state, the service of changing to one of those states will be degraded by the interruption in torque whereas a change to the other two drivetrain states would offer a better client service and better performance of the vehicle.